The present invention and is related to power brake boosters of the differential pressure type. In particular, the power brake booster of the present invention includes a simplified mechanical panic assist air valve assembly and a method of assembling such an air valve.
Power brake boosters of the differential pressure type using engine vacuum are well known in the motor vehicle industry. A brake booster of this type has a diaphragm separating a vacuum chamber open to a source of vacuum, such as created in an engine intake passage, and a working chamber normally provided with vacuum or atmospheric air by a valve apparatus responsive to input brake pedal actuation to provide brake assist force as required through a power piston fixed to the diaphragm and coupled to the booster output.
The valve apparatus in a standard vacuum booster is a double or bipartite valve arrangement. The valve may have a vacuum valve seat on the power piston that is engageable with a floating control valve as well as an air valve that also has a seat engageable with the same side of the floating control valve and is coupled to a brake input rod. In the rest position (no brake apply) the air valve is engaged with the floating control valve, and the vacuum valve is slightly separated therefrom to guarantee vacuum on both sides of the diaphragm and thus no brake boost assist. A brake input to the air valve moves the air valve seat axially past the vacuum valve seat, whereupon the floating control valve moves against the vacuum valve seat to shut off the flow of vacuum into the working chamber and allow the air valve seat to disengage and admit air at atmospheric pressure into the working chamber. The increase in air pressure in the working chamber produces a differential pressure across the diaphragm to move it axially; and the power piston moves with it to provide an assist force to the brake fluid in the master cylinder. Movement of the power piston allows the floating control valve to move toward the air valve seat; and the level of boost assist is thus matched to the driver""s input force. The opening of the air valve is calibrated to provide smooth brake apply in normal and typical driving situations.
Occasionally a driver may wish or need to stop the vehicle as quickly as possible. In such a situation, it might be possible to provide quicker brake application by admitting air into the working chamber of the brake booster at a faster rate. But this faster air admittance should not be provided except in such situations, since it would not produce appropriate braking behavior in normal driving. Linear solenoid and rotary motors have been used beneficially to provide dual rate actuation and control of brake boosters. However, the significant complexity of these systems can be a disadvantage in cost and reliability terms. Other brake boosters have been disclosed with the capability to provide a dual reaction ratio as a function of the input force using a dual rate spring-biased double valve. When the input force exceeds a predetermined force, the spring biasing force acting on the double valve is exceeded resulting in a collapse of the valve. Thus, this collapse causes an increased opening through the air valve seat providing a greater rate of airflow therethrough than during normal operation of the valve. The output gain of the booster is accordingly provided at an increased amount.
However, the attachment feature of the input and output portions of a prior art two-part valve can have some disadvantages. The attachment feature can be a screw attachment. There exists a potential for the screw attachment to unscrew, which affects the dimensional stack height of the valve and the function thereof and thus, the reliability of the valve. The attachment can be a pin. The integrity of the attachment and the alignment thereof can be compromised by an improper joining of the two-part valve as a result of a cross-threaded join of the two parts, or improper alignment before or after pinning or the like.
It would therefore be advantageous to provide a simple means for providing an input and an output portion of a valve for a brake booster using a minimum of robust parts that reduce or eliminate component misalignment and improper stack height in a reliable assembly that is easy to assemble.
The present apparatus provides a braking speed of application enhancement by increasing the air valve opening in a vacuum brake booster when a vehicle operator applies braking force quickly and forcefully. This is accomplished with a two-part air valve which is normally expanded axially to a normal length by an internal spring, but which may be shortened to open air valve by a greater amount when the brake pedal input force is sufficient to move the air valve axially against another spring into contact with a shoulder of the power piston. The internal spring has a much higher preload than the other spring so that the booster operates with a fully axially expanded air valve in normal, low force brake activation. Once the air valve axially engages the power piston, however, the increase in air valve opening is immediate and substantial with any further increase in braking force.
One aspect of the present invention provides an air valve assembly for a power piston in a vacuum brake booster including an input element with a hollow axial extension. The hollow axial extension includes a retaining portion. An output element is provided that includes an input extension slidably positioned within the hollow axial extension. The input extension includes an outer groove formed thereon and a ring member is disposed in the outer groove, the ring member positioned adjacent the retaining portion to prevent separation of the input element and outer element.
Another aspect of the present invention provides an assembly wherein the ring member extends outwardly from the outer groove to engage the retaining portion. The ring member can be a circlip.
The retaining portion of the input element can include an inwardly extending notch positioned at an open axial end of the hollow axial extension, an inwardly extending radial retention surface, an inwardly extending catch and an internal groove.
The groove can allow the clip to move axially with respect to the hollow axial extension. The hollow axial extension can include an open axial end. The open axial end can include a plurality of axial slots formed therein, the slots allowing portions of the open axial end defined thereby to be spread apart. The open axial end can include four slots formed in the open axial end. The open axial end can include a beveled surface to guide insertion of the input extension.
Other aspects of the present invention can include a flange formed on the input element. The flange includes a shoulder portion. A spring seat portion is formed on the output element and a spring is disposed between the flange and the spring seat to bias the input element and the output element apart from each other. The spring is provided with a predetermined load resistance above which the air valve assembly collapses by allowing the input element and output element to move axially toward each other. The valve assembly can include a pair of springs, the pair of springs compressing at different predetermined loads.
Another aspect of the present invention provides a method of assembling an air valve for a vacuum brake power booster. The air valve includes an input element and an output element. The method includes retaining a clip on an input extension of the output element. An axial extension of the input element is spread over the clip. The input extension of the output element is inserted into the axial extension of the input element and the input extension is retained to the axial extension. A spring is provided between the input member and the output member before the input extension is inserted into the axial extension.
Another aspect of the invention provides a vacuum brake power booster assembly including an air valve including a means for retaining a clip on an input extension of an output element of the air valve, a means for spreading an axial extension of an input element of the air valve over the clip, a means for inserting the input extension of the output element into the axial extension of the input element and a means for retaining the input extension to the axial extension. The assembly can further include a means for biasing apart the input element and output element. The biasing means compresses at a predetermined apply force to permit collapse of the air valve.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.